Re: [RFC 1/6] mm/migrate_pages: separate huge page and normal pages migration

["Huang, Ying" <ying.huang@intel.com>]

Hi, Zi,

Thank you for comments!

Zi Yan <ziy@nvidia.com> writes:

> On 21 Sep 2022, at 2:06, Huang Ying wrote:
>
>> This is a preparation patch to batch the page unmapping and moving for
>> the normal pages and THPs.  Based on that we can batch the TLB
>> shootdown during the page migration and make it possible to use some
>> hardware accelerator for the page copying.
>>
>> In this patch the huge page (PageHuge()) and normal page and THP
>> migration is separated in migrate_pages() to make it easy to change
>> the normal page and THP migration implementation.
>>
>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: Yang Shi <shy828301@gmail.com>
>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> ---
>>  mm/migrate.c | 73 +++++++++++++++++++++++++++++++++++++++++++++-------
>>  1 file changed, 64 insertions(+), 9 deletions(-)
>
> Maybe it would be better to have two subroutines for hugetlb migration
> and normal page migration respectively. migrate_pages() becomes very
> large at this point.

Yes.  migrate_pages() becomes even larger with this patchset.  I will
consider more about how to deal with that.  I will try the method in
your comments in [3/6] for that too.

Best Regards,
Huang, Ying

>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 571d8c9fd5bc..117134f1c6dc 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1414,6 +1414,66 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>
>>  	trace_mm_migrate_pages_start(mode, reason);
>>
>> +	for (pass = 0; pass < 10 && retry; pass++) {
>> +		retry = 0;
>> +
>> +		list_for_each_entry_safe(page, page2, from, lru) {
>> +			nr_subpages = compound_nr(page);
>> +			cond_resched();
>> +
>> +			if (!PageHuge(page))
>> +				continue;
>> +
>> +			rc = unmap_and_move_huge_page(get_new_page,
>> +						put_new_page, private, page,
>> +						pass > 2, mode, reason,
>> +						&ret_pages);
>> +			/*
>> +			 * The rules are:
>> +			 *	Success: hugetlb page will be put back
>> +			 *	-EAGAIN: stay on the from list
>> +			 *	-ENOMEM: stay on the from list
>> +			 *	-ENOSYS: stay on the from list
>> +			 *	Other errno: put on ret_pages list then splice to
>> +			 *		     from list
>> +			 */
>> +			switch(rc) {
>> +			case -ENOSYS:
>> +				/* Hugetlb migration is unsupported */
>> +				nr_failed++;
>> +				nr_failed_pages += nr_subpages;
>> +				list_move_tail(&page->lru, &ret_pages);
>> +				break;
>> +			case -ENOMEM:
>> +				/*
>> +				 * When memory is low, don't bother to try to migrate
>> +				 * other pages, just exit.
>> +				 */
>> +				nr_failed++;
>> +				nr_failed_pages += nr_subpages + nr_retry_pages;
>> +				goto out;
>> +			case -EAGAIN:
>> +				retry++;
>> +				nr_retry_pages += nr_subpages;
>> +				break;
>> +			case MIGRATEPAGE_SUCCESS:
>> +				nr_succeeded += nr_subpages;
>> +				break;
>> +			default:
>> +				/*
>> +				 * Permanent failure (-EBUSY, etc.):
>> +				 * unlike -EAGAIN case, the failed page is
>> +				 * removed from migration page list and not
>> +				 * retried in the next outer loop.
>> +				 */
>> +				nr_failed++;
>> +				nr_failed_pages += nr_subpages;
>> +				break;
>> +			}
>> +		}
>> +	}
>> +	nr_failed += retry;
>> +	retry = 1;
>>  thp_subpage_migration:
>>  	for (pass = 0; pass < 10 && (retry || thp_retry); pass++) {
>>  		retry = 0;
>> @@ -1431,18 +1491,14 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>  			cond_resched();
>>
>>  			if (PageHuge(page))
>> -				rc = unmap_and_move_huge_page(get_new_page,
>> -						put_new_page, private, page,
>> -						pass > 2, mode, reason,
>> -						&ret_pages);
>> -			else
>> -				rc = unmap_and_move(get_new_page, put_new_page,
>> +				continue;
>> +
>> +			rc = unmap_and_move(get_new_page, put_new_page,
>>  						private, page, pass > 2, mode,
>>  						reason, &ret_pages);
>>  			/*
>>  			 * The rules are:
>> -			 *	Success: non hugetlb page will be freed, hugetlb
>> -			 *		 page will be put back
>> +			 *	Success: page will be freed
>>  			 *	-EAGAIN: stay on the from list
>>  			 *	-ENOMEM: stay on the from list
>>  			 *	-ENOSYS: stay on the from list
>> @@ -1468,7 +1524,6 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>  						nr_thp_split++;
>>  						break;
>>  					}
>> -				/* Hugetlb migration is unsupported */
>>  				} else if (!no_subpage_counting) {
>>  					nr_failed++;
>>  				}
>> -- 
>> 2.35.1
>
>
> --
> Best Regards,
> Yan, Zi